The present invention is directed to an improved process for cleaning vessels containing sulfur dichloride. In the process of this invention, substantially all of the liquid sulfur dichloride is first removed from the vessel, and the material remaining in the vessel, including a slurry of liquid and solid material on the bottom, is heated in the presence of an inert gas to vaporize liquid components. The vapor is removed from the vessel by vacuum supplemented by the inert gas purge, neutralized, and the solid residue, primarily FeCl.sub.3, is then removed from the bottom of the vessel.
Sulfur dichloride, SCl.sub.2, is used extensively in the manufacture of organic chemicals and insecticides, as a vulcanizing agent for rubber, as a reagent, and as a lubricant additive. It is a brownish red liquid at room temperature which boils at about 60.degree. C. It spontaneously decomposes to form sulfur monochloride and chlorline at elevated temperatures.
Sulfur dichloride is conventionally prepared by the liquid phase chlorination of sulfur monochloride in the presence of a Lewis-acid catalyst such as FeCl.sub.3. Typical methods of preparation are disclosed in U.S. Pat. No. 3,219,413 to Kunkel et al. and U.S. Pat. No. 3,205,040 to Schmadebeck.
Sulfur dichloride is commercially shipped in liquid form in steel railroad tank cars. The sulfur dichloride is removed from the tank car through an opening in the top of the car leaving a mixture of liquid sulfur dichloride and solid material on the bottom of the tank car. This solid material is primarily ferric chloride with smaller amounts of ferrous chloride and ferrous sulfide. A residue of this solid material also coats the internal surfaces or walls of the tank car. Following normal shipping practice, only the sulfur dichloride is removed, and the tank car is then put back into service.
Vessels such as tank cars must be periodically static pressuretested for leaks to meet regulatory requirements and to insure safe operating conditions. Prior to such testing, the vessel must be completely cleaned of reactive materials. Present cleaning techniques involve the addition of water or aqueous solutions of sodium hydroxide to the vessel. The water or sodium hydroxide solution reacts with the sulfur dichloride to form hydrochloric acid, chlorine, sulfur and sulfur dioxide. These reaction products are, in general, less reactive and easier to handle than sulfur dichloride. Hydrochloric acid, which is the primary reaction product, can be neutralized with additional sodium hydroxide and safely removed from the vessel.
This procedure is subject to numerous disadvantages. The reaction of water or an aqueous sodium hydroxide solution with the contents of the vessel is strongly exothermic and forms gaseous reaction products (HCl and SO.sub.2). These reactive gases are produced at a rapid rate which is not easily controllable. In addition, since the reaction is exothermic, the temperature inside the vessel can increase dramatically and cause a substantial pressure build-up. These reaction products are also highly corrosive and cause severe corrosion to the internal surface of the vessel.
Consequently, it is desirable to develop an improved process for cleaning vessels containing sulfur dichloride which is safer and more convenient than previous cleaning methods.